1. Field of the Invention
This invention relates to amplifiers and, more particularly, to a pilotless feed forward distortion reduction system.
2. Description of Related Art
Amplifiers often add undesired distortion to a signal, creating an output signal comprising distortion or nonlinear components and the signal component. The distortion includes any undesired signals added to or affecting adversely the input signal. There is therefore a need to devise techniques that can eliminate substantially or reduce significantly the distortion produced by the amplifier.
Feed-forward correction is routinely deployed in modem amplifiers to improve amplifier linearity with various input patterns. The essence of the feed-forward correction is to manipulate distortion, such as intermodulation (IMD) components, created by the amplifier so that at the final summing point, the distortion cancels out. Due to the unpredictability of input RF carrier pattern as well as the resultant distortion location, a known frequency component, i.e. a pilot signal, is injected in the main signal path to mimic the distortion produced by the amplification process. In feed-forward amplifiers, the feed forward distortion reduction circuitry minimizes the pilot signal along with the distortion. As such, by designing the feed forward distortion reduction circuitry to detect and cancel the pilot, the distortion can also be removed.
The pilot signal is an electrical signal comprising at least one frequency component spectrally located near the frequency band of operation of the electrical circuit. A more complete description of the pilot signal is shown in FIG. 1 which shows the frequency response of a radio frequency (RF) amplifier including the location of the pilot signal. The pilot signal can be near the lower edge of the operating band (e.g., pilot 1) or located near the upper edge of the band of operation (e.g., pilot 2). Tie pilot is positioned a spectral distance of .DELTA. .intg. from an edge of the band of operation whose center frequency is .intg..sub.0. The pilot signal can also be located somewhere within the band of operation of the RF amplifier. The electrical characteristics (e.g., amplitude, phase response, spectral content) of the pilot signal are known. It should be noted that although the pilot signal is shown as a single spectral component of a certain amplitude, the pilot signal can comprise a plurality of spectral components having various amplitudes.
The feed forward distortion reduction circuitry reduces distortion produced by the RF amplifier by applying the pilot signal to the RF amplifier and making adjustments based on information obtained from the applied pilot signal. FIG. 2 discloses feed-forward correction circuitry 10 and its use of information obtained from the pilot signal to reduce distortion produced by RF amplifier 12. An input signal is applied to a splitter 14. The splitter 14 replicates the input signal on a main signal path 16 and a feed forward path 18. The splitter 14 is part of a feed forward loop referred to as loop #1, which in addition to the splitter 14, comprises gain & phase circuit 20, coupler 22, the RF amplifier 12, delay circuit 24 and couplers 26 and 28. The signal on the main path 16 is applied to gain & phase circuit 20. The output of gain & phase circuit 20 and the pilot signal are applied to the coupler 22. Typically, the amplitude of the pilot signal is much less (e.g., 30 dB less) than the amplitude of the input signal so as not to interfere with the operation of the amplifier 12. The output of coupler 22 is applied to the amplifier 12 whose output comprises the amplified input signal, the amplified pilot signal and distortion signals produced by the amplifier 12. A portion of the output of the amplifier 12 is obtained from the coupler 26 and is combined with a delayed version of the input signal (signal on path 18) at the coupler 28 via coupling path 30. The signal on the path 18 has experienced sufficient delay provided by delay circuit 24 so that such signal experiences the same delay as the signal appearing at the coupler 28 via the path 30.
The gain & phase circuit 20 is controlled via control path 32 with control signals to adjust the gain and phase of the signal such that the signal appearing at the coupler 28 via the path 30 is substantially the inverse (equal in amplitude but 180.degree. out of phase) of the delayed input signal at the coupler 28. The control signal appearing on the control path 32 of the gain & phase circuit 20 is derived from the signal at point A in a well known manner such as the use of detection circuits. The detection circuits detect well known electrical signal characteristics such as amplitude, phase, and frequency of the signal. Therefore, the input signals applied to the coupler 28 substantially cancel each other leaving at point A the pilot signal and the intermodulation distortion signals produced by the amplifier 12. Loop #1 is thus a feed forward loop which serves to isolate at point A the pilot signal and distortion signals produced by the amplifier 12.
The signals appearing at point A (pilot signal and distortion signals) are fed to gain & phase circuit 34 whose output is fed to amplifier 36 whose output is applied to coupler 38. A portion of the output signals (signal, pilot signal and distortion signals) of the amplifier 12 is fed to delay circuit 40 whose output is fed to the coupler 38. The delay circuit 40 is designed such that signals from the output of the amplifier 12 applied to the coupler 38 experience substantially the same delay as the signals from the output of the amplifier 36 applied to the coupler 38.
Because the frequency, amplitude and other electrical characteristics of the pilot signal are known, pilot detect circuit 42 can use detection circuits such as a mixer connected to a log detector (or other well known detection circuits) to detect the pilot signal or a portion of the pilot signal via coupler 44. The pilot signal is used to obtain information about the distortion produced by the amplifier 12. The information is obtained by detecting well known electrical signal characteristics of the pilot signal. In particular, the characteristics (e.g., amplitude, spectral content, phase response) of the pilot signal are known and thus when the pilot detect circuit 42 detects the pilot signal, some or all of the known characteristics can be used to isolate the pilot signal. Detection circuit 42 will detect the pilot signal and use this information to generate control signals onto path 46 to cause the gain & phase circuit 34 to modify the pilot signal at point A such that the pilot signal on the main path 16 at the coupler 38 is substantially the inverse (equal in amplitude but 180.degree. out of phase) of the pilot signal on the feed forward path 18 at the coupler 38. The corresponding pilot signals and the distortion signals at the coupler 38 substantially cancel each other at the coupler 38 leaving the signal (or an amplified version of the signal) at the output of the system. Therefore, loop #2, which comprises the coupler 26, the coupler 28, the gain & phase circuit 34, the amplifier 36, the coupler 38 and the delay circuit 40 is a feed forward loop which uses the information obtained from the pilot signal to cancel substantially the distortion produced by the amplifier 12.
In actual systems, however, there is rarely an absolute cancellation of the distortion and the pilot signals. Feed forward distortion reduction systems require tight operating tolerances, for example, to achieve a 30 dB reduction in IMDs, typical feed forward correction systems may require a + or -1 dB frequency flat response (amplitude deviation over the frequency band of operation) and a + or -1 degree phase linearity (phase deviation from a straight line in the frequency band of operation). To obtain this accuracy is difficult.
The amplitude of the pilot signal is typically relatively small at the output of the feed forward distortion reduction system because of the cancellation of the pilot and the relative amplitude of the pilot signal with respect to the amplitude of the output signal. Thus, it becomes difficult to detect the pilot signal at the output of the system. To improve detection of the pilot signal at the output of the correction circuitry, schemes are developed to place the pilot signal at an appropriate location and to improve detection.
Thus, a need exists for a distortion reduction system that can provide adequate distortion reduction while eliminating the need for the pilot signal to reduce distortion.